1. Field of the Invention
The present invention relates to novel zeolites based on silica and germanium oxide, and, more especially, to novel zeolites having an MFI structure and to a process for the synthesis thereof.
2. Description of the Prior Art
Zeolites are crystallized tectosilicates. Their structures are aggregates of TO.sub.4 tetrahedrons defining a tridimensional skeleton by the sharing of oxygen atoms. In the aluminosilicate type zeolites, which are the most common, T represents tetravalent silicon and trivalent aluminum. The cavities and channels of molecular dimensions in this skeleton accept cations which compensate for the charge deficit due to the presence of trivalent aluminum in the tetrahedrons. Trivalent elements, such as gallium and more rarely boron or beryllium, may be substituted for the aluminum.
In general, the composition of the zeolites may be represented by the overall formula of M.sub.2/n O; Y.sub.2 O.sub.3 ; xZO.sub.2 in the dehydrated and calcined state. Z and Y respectively represent the tetravalent and trivalent elements of the TO.sub.4 tetrahedrons; M is an electropositive element having a valence n, such as an alkali or alkaline earth metal element constituting the compensating cations; x may range, theoretically, from 2 to infinity, in which case the zeolite is a silica.
Each type of zeolite has a distinct pore structure. The variations in the dimensions and shapes of the pores from one type of zeolite to another are the principal factors contributing to the differences in the adsorbent properties thereof. Only molecules having certain dimensions and shapes are able to penetrate into the pores of a particular zeolite. In light of their remarkable properties, the zeolites are especially suitable for the purification or separation of gaseous or liquid mixtures, such as, for example, the separation of hydrocarbons by selective adsorption.
The chemical composition and in particular the nature of the elements present in the TO.sub.4 tetrahedrons and the nature of the exchangeable compensating cations, are also important factors affecting the selectivity of the adsorption, and particularly the catalytic properties of these materials. They are used as catalysts or catalyst supports in the cracking, reforming and modification of hydrocarbons, as well as in the synthesis of numerous molecules.
Many zeolites exist in nature; these are aluminosilicates, the availability and properties of which do not always satisfy the requirements of industrial applications. For this reason, considerable research has been carried out in quest of products having novel properties. This has resulted in the synthesis of a large variety of zeolites, essentially of the aluminosilicate type. Among the numerous examples of this type, the following are representative: zeolite A (U.S. Pat. No. 2,882,243); zeolite X (U.S. Pat. No. 2,882,244); zeolite Y (U.S. Pat. No. 3,130,007); zeolite L (FR 1,224,154); zeolite T (FR 1,223,775); zeolite ZSM5 (U.S. Pat. No. 3,702,886); zeolite ZSM12 (U.S. Pat. No. 3,832,444); zeolite ZSM48 (EP 0,015,132).
Synthetic zeolites containing germanium in the TO.sub.4 tetrahedrons are also known to this art. Tetravalent germanium may be either partially or completely substituted for the tetravalent silicon.
A trivalent element, such as aluminum or gallium, is thus always present in the tetrahedrons, in addition to the tetravalent elements. The following combinations, containing germanium in the TO.sub.4 tetrahedrons of the skeleton are known to the art: (Si.sup.IV, Ge.sup.IV, Al.sup.III); (Si.sup.IV, Ge.sup.IV, Ga.sup.III); (Ge.sup.IV, Al.sup.III); and (Ge.sup.IV, Ga.sup.III). In this respect, compare the publication by R. M. Barrer, J. W. Baynham, F. W. Bulktude and W. M. Meier in J. Chem. Soc. (1959) and the patents relating to faujasite (BE 798,818), NU-27 (EP Application No. 131,320), EU-7 (EP Application No. 107,908), EU-13 (EP Application No. 108,486), NU-10 (EP Application No. 77,624), NU-5 (EP Application No. 54,386), NU-6 (EP Application No. 54,364), NU-2 (EP Application No. 55,046), FU-9 (EP Application No. 55,529), NU-13 (EP Application No. 59,540), NU-3 (EP Application 40,016), SSZ-15 (U.S. Pat. No. 4,610,854), ZSM-5 (U.S. Pat. No. 3,702,886), ZSM-5 (EP Application No. 34,727) and ZSM-5 (FR 2,472,538).
Zeolites in which the silicon is substituted by germanium in the absence of trivalent elements, i.e., the TO.sub.4 tetrahedrons contain only the (Si, Ge) couple, have not to date been proposed to this art.
The zeolites are typically prepared from a reaction mixture which is converted in a hydrothermal medium and by a dissolution/recrystallization process, the crystalline precipitate which results being calcined after separation and drying, to yield an active zeolite.
The reaction mixture contains reagents designed to supply the elements T to be incorporated into the skeleton of the zeolite, such reagents typically being aqueous gels containing oxides or hydroxides of the elements T.
The reaction mixture also contains one or more mobilizers favoring the dissolution of said reagents and transfer from the aqueous phase to the zeolite crystals being formed, and a structuring agent which gives rise to the formation of micropores, and which also stabilizes the zeolite.
If OH.sup.- anions are used as the mobilizing agent, the reaction media are characterized by a basic pH, typically higher than 10. These media are well suited for the dissolution of starting materials containing the elements of silicon and aluminum, and, in general, all of the elements yielding oxygen anions soluble in a basic medium.
If strong bases are used, such as the hydroxides of alkali metals, highly supersaturated media are obtained which makes possible the rapid crystallization of zeolites. But it is often difficult to control the formation of the desired crystallized phase, which, in most cases, is metastable. On the other hand, a high rate of crystallization may lead to the formation of defects in the skeleton of the TO.sub.4 tetrahedrons, such as --T--O.sup.- instead of --T--O--T--. Finally, the presence of compensating alkaline cations in the channels and cavities is frequently detrimental in certain applications; it may then be necessary to conduct an ion-exchange with other cations. The latter disadvantage may be avoided by replacing these bases with other strong bases, such as alkylammonium hydroxides, which may simultaneously act as structuring agents. However, the high cost of these materials limits their use on an industrial scale. The use of weaker bases, such as the amines, which also have structuring properties, largely eliminates the aforementioned difficulties. But in this case, the concentration in OH.sup.- mobilizer anions may become too low, which leads to excessively slow reaction rates.
It is also possible to employ fluoride anions F.sup.- as a mobilizing agent of elements yielding soluble fluorine complexes.
The usable pH range is extended in this case toward neutral or even acidic pH values. The selection of cations that may be used in the reaction medium is also larger, as it may include cations such as NH4.sup.+, which are easily eliminated after the synthesis by calcination.